Wave amplification



Dec. 5, 1939. L. c. PETERSON 3 WAVE AMPLIFICATION Filed Dec. 4, 1937 2SheetsShee t l FIG/,4

//vv/v TOR L. C. PETERSON ATTORNEY Patented Dec. 5, 1939 PATENT 1owners:

2,181,910 I I WAVE. AMPLIFICATION Liss 0. Peterson, Madison, N. J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application December 4, 1937, Serial No.178,058

7 Claims.

The present invention relates to wave translation circuits employingspace discharge devices for repeating waves and more especially foramplifying waves while minimizing distortion.

' The general object of the invention is an amplifier of particularconstruction for amplifying waves with a reduced amount of accompanyingdistortion.

More specifically the invention relates to amplifiers employing tubeswith two gridsthe usual negatively biased control grid and a positivelybiased space charge grid or netwith suitably constructed circuits forsecuring increased gain and reduced distortion.

Reduced distortion has more recently been associated with lowered gainin the so-called stabilized feedback amplifiers. With this invention,however, an increase in gain is accompanied, under certain circumstancesto be outlined, with decreased distortion. An additional improvement indistortion ratio can, however, be realized, in accordance with theinvention, by including amplifier stages of increased gain and lowereddistortion in a circuit having over-all negative or gain-reducingfeedback.

The various objects and features of the invention will be more fullyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings in which Figs. 1, 1A and 2 show schematiccircuit diagrams of typical amplifier circuits illustrative of theinvention, and Figs. 3 to 6 show illustrative performance curves, aswill presently be more fully described.

Referring first to Fig. 1, a three-stage amplifier is shown havingamplifying stages 10, H and I2, each comprising a tube having a cathodel, space charge grid or net 2, control grid 3, screen grid 4 and anode5. The input is at 13 and output at 14. A common source is indicated(but not shown in detail) for supplying positive potential to the netsof all stages through connections including choke coils I5, l6 and H. Acommon voltage source supplies screen grid voltage to all stages throughchoke coils I8, 19 and 20. Space current is supplied to stages 10 and 11through choke coils 21 and 22, respectively, in series with couplingimpedance networks 23 and 24'. By-pass condensers are shown between allof these supply connections and ground. The space current supply fortube 12 is assumed to be through the load to be connected in suitablemanner at 14 but not actually shown. Negative bias for the control gridsis furnished from bias resistors 23, etc., which have by-pass condensersas usual. Interstage series coupling condensers are shown at 24 and 25;grid leaks at 21, 28 and alternating current net-to-cathode connectionsare shown at 29, 39 and 31' which, besides the stopping condensers, maycomprise simple re- '5' sistances as in Fig. 1 or reactances orcomplexnetworks of suitable type, one form being shown in Fig. 1A forillustration. An input circuit shown only in part extends throughsuitable in- U put source or coupling at [3. l0

The circuit in Fig. l is shown as supplied with an over-allgain-reducing feedback path from an output circuit resistor 30 throughstopping condenser 3| and resistor 32 to input resistor 33.

In accordance with a feature of this invention, 15 a coupling impedanceis connected between the control grid and space charge grid or net,comprising in Fig. 1 the condenser 35 of the first stage andcorresponding condensers 3B, 31 of later stages. 20

As pointed out in a copending application of K. C. Black Serial No.86,778, filed June 23, 1936,

a suitable impedance suitably coupled between the control grid and netincreases the gain of an amplifier stage materially. The condensers 35,36 25 and 31 shown in Fig. I serve to increase the gain in the mannertaught by Black by increasing the control grid-to-cathode impedance dueto the effect of negative transconductance from control grid to net, asreferred to below. They also, 30 by virtue of the resulting feedbackbetween control grid and net, serve to reduce materially the shuntcapacity across the input terminals.

I have discovered, in accordance with this invention, that under certainconditions it is pos- 35 sible to obtain, along with the increased gainand lowered shunt capacity as disclosed by Black, a material reductionin distortion. Thus, apparently, for the first time there are secured inone and the same circuit increased gain and 40 increased purity ofoutput. 7

These effects are dependent upon particular tube characteristics whichapplicant has found by test exist in certain available types of tube,such as Western Electric types '75'75F and 291A and 45 the RCA type 6A7.In each of these there is negative transconductance from control grid tonet, taking the transconductance from control grid to anode as positive.Where the external impedance 29 between net and cathode is so high thatthe internal impedance between these elements is controlling, thetransconductance of the amplifier stage is at maximum value and made upof two parts, first, that from control grid to anode and, second, theproduct of the transconductance from net to anode and the amplificationfactor from control grid to net. These are additive in their effects.While this completely determines the maximum gain obtainable, there isan additional factor determining the distortion.

As is well known, the anode current wave of a grid-controlled tube isnot an exact replica of the input voltage wave but contains certaindistortion products which can be expressed mathematically'as terms in apower series. These terms first appear in the expression for the platecurrent; they are not present in the expression for the grid wave, whichmay, for example, be of simple sinusoidal form. Considering now that thenet-cathode space path" is also a currentcarrying path in which thecurrent is a function of the control grid voltage, it occurred toapplicant that use might be made of the distortion products in the netcurrent to oppose the distortion products in the anode current andreduce the magnitude of the latter. One possible explanation to accountfor this action is as follows. A convenient convention often used toaccount for plate current distortion in a triode is to assume first thatthe tube has a prefectly linear amplifying characteristic and then toassume the existence of a fictitious source of distortion voltage atsome point in the circuit. This fictitious source may be assumed in theplatecathode circuit or in the. grid-cathode circuit, having due regardto magnitude and sign. With a proper shape and magnitude of distortionvoltage such an ideal circuit would produce plate current containing thesame signal and distortion components as occur in the case of an actualtriode. Conversely, a fictitious source of distortion voltage, forexample in the grid circuit of a triode, can neutralize or reduce theplate current distortion provided the shape, magnitude and sign of thefictitious voltage are properly chosen.

Now if we have a multigrid tube in which amplification is producedbetween voltage waves applied to the negative grid and the resultingvoltage waves in the net circuit the distortion prod ucts in suchcurrent may be thought of as arising from a fictitious generator ofdistortion voltage situated between the cathode and net. But it is clearthat variations in the net potential are repeated into the anode circuitin amplified form because of the transconductance factor fromnet-to-anode circuits. For example, in the simple case of second orderdistortion a second order type of fictitious voltage of proper magnitudeand sign in the net circuit would neutralize or reduce second orderplate current distortion. In order to neutralize or reduce some othertype of plate current distortion the corresponding type of fictitiousvoltage must be assumed in the net circuit.

The warrant for assuming a fictitious distortion voltage in the netcircuit is based in reality on the fact that the static characteristicof the net-cathode circuit is non-linear, and the kind of non-linearitydetermines the type of distortion. It follows then that if thenon-linearity of the static characteristic of the net-cathode circuit issuitably related to the non-linearity of the static characteristic ofthe anode-cathode circuit the resulting distortions in the two circuitsmay be made to oppose each other. A necessary condition for this tohappen is that the static characteristics of the plate circuit and ofthe net circuithave similar shape in the region of the operating point.Applicant has found that under these conditions a material reduction inplate current distortion is realized.

For example, referring to the curves of Figs.

3, 4 and 5, which are mostly self-explanatory,

it was found that when the net-to-cathode circuit was notshort-circuited (by-passed) there fragment of the net characteristic andthe portion .of the plate characteristic opposite which it is placed.That is, the signal applied to the control grid produced a voltage swingof i 1 Volt maximum value in the net voltage and a voltage swing ofabout 15 volts maximum in the plate voltage. Both characteristics hadslight downward curvature throughout the operating range, hence were ofthe same general shape throughout the operating range.

For these curves, a single stage similar to one of the stages of Fig; 1was employed, (without, of course, the external negative feedback path30 to ,33); A fixed capacity of 98.2 micro-microfarads was used forcondenser 35 and the plate load was 6000 ohms, the tube being WesternElectric 757,51 type. 'Itwill'be noted that the curve in Fig. 4indicates a minimum point for second harmonic corresponding to a 'valueof external net impedance about half the internal net impedance. Thecurve is left broken in this region since its exact shape was notdetermined in the vicinity of the minimum point.

The circuit of Fig. 2 differs from the circuit of Fig. 1 only in themanner in which the overall external feedback path is connected. In Fig.2 the feedback is brought to a point in the net circuit instead of inthe control grid circuit.

It was pointed out above that it is advantageous to have thevnet-to-plate'.transconductmice as large as possible for largestincrease in gain, largest reduction in shunt capacity and largest.improvement in distortion. When this condition holds, it is alsoadvantageous to'use the net for the negative feedback connection'toenable large reduction in gain to be realized and at thesame'tir'n'eia'nd in any case to) keepthe feedback path independent ofthe input circuit,

thereby avoiding the unfavorable impedance relations that might exist insome cases if the feedback path were connected directly to the con trolgrid. The Fig. 2 type of connection also eliminates from the. feedbackpath some parasitic elements such as shunt capacity of input transformeror otllg apparatus connected to the ee e s i One of the importantadvantages achieved by the invention is the reduction indistortio n bythe net-to-grid coupling already described. For a given gain andsignal-to-distortionratio in an amplifier such, for example, Iasisillustrated in Fig. 1 or Fig. 2, the reduction in distortion inindividual stages eases the requirements placed on the characteristic ofthe amplifier as a whole, thatis, on the gain and phase requirements ofthe amplifier-feedback loop; As is known, in broad band negativefeedback amplifiers a singing tendency develops at high frequencies(generally much higher than the utilized range) due to the shuntimpedances becoming predominantly capacitive. 1f the phase changessignbefore thegain fallstozero as the upper limiting frequency isapproached, singing will be very likely, if not certain, to occur. Thereduction of distortion in individual stages permits use of a smallerover-all negative feedback for a given distortion level and this isequivalent to raising the frequency at which the phase changes sign,making the amplifier less liable to develop singing. This margin inphase shift relaxes the design requirements of the amplifier-feedbackloop. This advantage is, of course, in addition to the advantages ofincreased gain and lowered shunt capacity already pointed out asresulting from the net-to-control grid coupling.

What is claimed is:

1. In combination, a space discharge device having a cathode, an anodeand a control grid, means to impress signal voltages on the controlgrid, an anode-cathode circuit into which said signals are repeated witha certain amount of anode modulation, and means reducing such modulationcomprising a net biased positive and connected to the cathode by anexternal impedance and having means to impress signal voltages on saidnet, the static characteristics of said net-cathode circuit and saidanode-cathode circuit simulating each other sufiiciently within theutilized range to reduce the amount of such modulation.

2. I'he combination according to claim 1 in which said means to impresssignal voltages on said net comprises a coupling between said controlgrid and said net such as to increase the gain of the amplifier.

3. The combination according to claim 1 in which a feedback circuit isconnected from a point on the output side of said discharge device tosaid net to feed back some of the output voltage in gain-reducing senseto reduce the modulation present in the anode-cathode circuit.

4'. A multistage amplifier, each stage of which comprises thecombination according to claim 1 and in which a negative feedbackcircuit is connected from the anode circuit of a later stage to the netof a preceding stage, such feedback circuit reducing the gain of theamplifier and reducing the total modulation. v

5. In an amplifier circuit, a space discharge device having a cathode,an anode, a control grid, a second grid, biased positive toward thecathode, said tube exhibiting gain from the second grid to the anode andfrom the control grid to the anode, and having negative transconductancefrom the control grid to the second grid, and means impressing signalson both of said grids, the static characteristics of the secondgrid-tocathocle path and of the anode-to-cathode path being of similarshape throughout the operating region whereby the modulation is lowered.

6. In an amplifying circuit, a space discharge device comprising acathode, an anode, a control grid and a second grid biased positivetoward the cathode, said tube having negative transconductance from thecontrol grid to the second grid, means for increasing the gain of saidamplifier comprising a coupling between said two grids, and meanslowering the modulation in the output of said amplifier comprising meansfor producing static characteristics of similar shape for theanode-cathode circuit and for the second grid-to-cathode path in theregion of the operating point whereby the modulation products set up insaid second grid-to-cathode circuit reduce the magnitude of themodulation products in the anode-cathode circuit.

7. In an amplifier circuit having an input and an output, a spacedischarge device comprising a cathode, an anode, a control grid and aspace charge grid, said control grid being connected to receive wavesfrom said input, means to impress waves from said anode on said output,means to impress waves from said input on said space charge grid in suchphase as to increase the gain of said circuit, and a gain-reducingfeedback connection from said output to said space charge grid.

LISS C. PETERSON.

